DES MOINES, Iowa — Paul Grinvalds has spent decades chasing the finish line, completing 114 marathons across all 50 states. Yet, for the 68-year-old West Des Moines resident, the greatest adversary is no longer the daunting 26.2-mile distance or the hilly terrain of a difficult course. It is the thermometer. Even on days when the mercury hits a modest 60 degrees Fahrenheit (15.5 Celsius), Grinvalds feels the toll of the heat, which forces his body into a physiological struggle to maintain equilibrium.
"One thing about marathoners is that we’re kind of stubborn," Grinvalds admits. "If we have a goal and it’s hot, a lot of times we’re still going to try to do that goal, which is just stupid because you’re almost guaranteed to have a very painful experience."
Grinvalds is not alone. Across the United States, the marathon community—a culture defined by grit, perseverance, and rigid training schedules—is facing a harsh, new reality. As climate change, fueled by the accumulation of greenhouse gases in the atmosphere, pushes global temperatures to record-breaking highs, the sport of distance running is undergoing a fundamental transformation. Race directors are moving dates, reconsidering start times, and forced to make the agonizing decision to cancel events entirely to protect the lives of their participants.
The Shrinking Window: A Chronology of Climate-Driven Disruption
The impact of rising temperatures on endurance sports has accelerated significantly over the last few years. The trend is not merely anecdotal; it is a systemic shift in the logistics of public sporting events.
The turning point for many organizers came in 2023, when the Twin Cities Marathon in Minneapolis-St. Paul was forced to cancel at the eleventh hour. The decision, deemed "devastating" by race director Ed Whetham, was a response to record-breaking heat and humidity that created an untenable risk for thousands of runners. The event served as a wake-up call for the industry, prompting a comprehensive re-evaluation of historical weather data.
"We had to look at the patterns," says Whetham. "We realized that the traditional first Sunday in October was no longer the reliable window of crisp, cool autumn air we had enjoyed for forty years."
Following an extensive analysis of regional climate shifts, the Twin Cities Marathon organization announced a historic change: beginning in 2027, the race will be permanently shifted to later in October. While this carries the risk of colder, potentially snowy conditions, organizers are prioritizing safety. "We’ll take a 39-degree (4 C) start over a 70-degree (21 C) start any day," Whetham notes.
This is a nationwide pattern. The Sioux Falls Marathon in South Dakota followed a similar trajectory, moving its annual race from August to September after a total cancellation in 2024 due to extreme heat. Other iconic races, including the Air Force Marathon in Ohio, the Little Rock Marathon in Arkansas, and the Long Beach Marathon in California, have shifted their start times to the pre-dawn hours, desperate to keep runners off the course when the sun—and the temperature—reach their peak.
Physiological Warfare: Why Heat Defeats the Body
The biological challenge of running a marathon in heat is a matter of basic physics and fluid dynamics. Dr. Chris Minson, a physiologist specializing in thermoregulation, explains that human performance is constrained by the body’s inability to simultaneously fuel muscles and cool the blood.
"As you get to warmer temperatures, you end up having a bigger competition between blood supplying your muscles to keep running and blood flow to your skin for thermal regulation," Minson explains.
The math of exertion is unforgiving. A runner’s body is remarkably inefficient; only about 25% of the energy produced during a run is used to propel the body forward. The remaining 75% is converted into heat. In cooler weather, this heat is easily dissipated through radiation and sweat evaporation. However, when the ambient temperature rises into the 60s and 70s, or when humidity traps moisture against the skin, the cooling mechanism fails. The heart is forced to work exponentially harder to pump blood to the periphery to cool the body, often at the expense of oxygen delivery to the working muscles.
Supporting Data: A Warming World and Declining Performance
The anecdotal evidence from runners is backed by stark climate data. The contiguous United States recently experienced its warmest July on record, and 2024 was globally the hottest year ever documented. Projections from the World Meteorological Organization suggest that the average global temperature will likely rise by more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels between 2025 and 2030.
Climate Central, a research group that analyzes the intersection of climate and daily life, studied 221 marathons across the country to determine how rising temperatures affect the "optimal" window for performance. Their findings were sobering. By 2045, 46 of the 51 marathons analyzed will likely lose the ability to provide the ideal race-day temperature—the low 40s Fahrenheit.
"The ability to go outside, to exercise, to take advantage of your human body and being healthy—that’s something that’s getting challenged by climate change," says Andy Pershing, who led the research at Climate Central.
Official Responses and Safety Measures
Race organizers are under increasing pressure to adapt. In March 2025, the Los Angeles Marathon took a controversial step: they offered participants the option to stop at the 18-mile mark and still receive a finisher’s medal. While the initiative was designed to keep runners off the course during the hottest part of the day, it was met with some derision from the traditionalist wing of the running community.
Meg Treat, a spokesperson for the event, defended the decision as a triumph of safety over ego. "We welcome the feedback, and it’s certainly something that caused a lot of discussion," Treat said. "But when we’re grading ourselves on it, it has to do with how safe our runners were on the day of the event, and we’re very happy with the outcome." Less than 5% of the 27,000 runners opted to stop early, and hospitalizations were kept to a minimum—a statistic the organizers credit to the flexibility of their safety policies.
Jessica Murfree, who leads a laboratory at the University of North Carolina-Chapel Hill dedicated to studying the impact of climate change on sports, believes these adaptations are merely the beginning. She argues that the issue extends beyond marathons to all outdoor athletics, including football and tennis.
"If we can’t walk, if we can’t exercise or go for runs, it’s too hot, can we still stay physically active while remaining cool?" Murfree asks. Her research suggests that society must prepare for a future where outdoor exercise becomes a luxury, or at least a highly regulated activity, as heatwaves become more frequent and volatile.
Implications for the Future: A Cultural Shift
The cultural shift in running is not just about race logistics; it is about the mentality of the athlete. Douglas Casa, a professor of kinesiology and a leader at the Korey Stringer Institute, which tracks heat-related deaths in sports, emphasizes that individual responsibility is as critical as organizational policy.
"If someone was smart enough to say, ‘OK, it’s a little hotter today than usual, I’m going to run a 7-minute mile instead of a 6:30 mile,’ then they won’t get any hotter than normal," Casa says. "It’s just that runners are sometimes very locked into what they normally do and what their goals are."
For athletes like John Seals, also of West Des Moines, the heat is a variable to be conquered through extreme preparation. Seals trains for the Badwater 135, an ultramarathon through Death Valley, by intentionally running in the midday sun and utilizing sauna sessions to induce heat acclimation. "We had a heat wave this week and for me, that’s a great thing," he says, viewing the rising temperatures as a training tool rather than a deterrent.
However, the consensus among experts is clear: relying on human grit to overcome rising global temperatures is a losing strategy. As the climate continues to change, the definition of a "successful" race is being rewritten. Success is no longer measured solely by a personal best time, but by the ability of the community to endure the heat, keep its participants safe, and adapt to a planet that is fundamentally warmer than the one where these traditions were first established.
The marathon, once a symbol of human endurance against distance, has become a symbol of human endurance against the environment itself. As organizers and runners look toward the 2030s and beyond, the message from the finish line is clear: change is no longer optional—it is the only way to keep the race going.
